1. Field of the Invention
The present invention particularly relates to an optical recording medium tilt compensation device having a tilt compensation unit able to compensate for a tilt error in servo control used when recording or reproducing data in an optical recording medium, a tilt compensation method, and an optical information recording apparatus using this method.
2. Description of the Related Art
A well known data recording method involves recording data on or reproducing data from an optical recording medium using light. A commercialized DVD system is a typical example of the optical recording method, which has been developed to meet the needs of recording image data compressed by MPEG2 equaling to two or more hours on one side of an optical recording medium 12 cm in diameter. In the DVD standard, the storage capacity of one side of the disk is 4.7 GB, the track density is 0.74 μm/track, and the line density is 0.267 μm/bit. Below, a DVD disk in compliance to this standard is referred to as “current DVD”.
An optical head is used to reproduce data recorded on the optical recording medium, such as a DVD. In an optical head, by an objective lens, a laser beam from a laser diode is condensed on a series of pits in tracks on the optical recording medium. The light beam reflected from the optical recording medium is condensed on an optical detector by a condensing lens, thereby producing a reproduction signal. The reproduction signal from the optical detector is input to a reproduction signal processing system to perform demodulation. For recording media conforming to the DVD standard, the wavelength of the laser diode in the optical head is 650 nm, and the numerical aperture of the objective lens (NA) is 0.6.
In addition, a Blu-ray Disc standard is known as a high density DVD standard, which is expected to be a large capacity optical recording medium video recorder standard of the next generation. Specifically, at most 27 GB of video data can be recorded and reproduced repeatedly on one side of a phase change optical recording medium, which has the same 12 cm diameter as the CD and the DVD. With a Blu-ray Disc, although a blue laser of a short wavelength is used, by setting the numerical aperture of the objective lens (NA) for condensing the laser to be 0.85, the beam spot can be made tiny.
If using an optical recording medium including a light transmission protecting layer having a thickness of 0.1 mm, which meets the requirement of increased numerical apertures of lenses, the aberration caused by tilt of the optical recording medium can be reduced, and it is possible to reduce malfunctions in read operations or improve the recording density. For this reason, by reducing the recording track pitch of an optical recording medium to as small as 0.32 μm, which is only about half of the track pitch of a DVD, a high storage capacity of 27 GB on one side of an optical recording medium has been put in practical use.
FIG. 15 is a schematic view of an optical pickup capable of writing data in a DVD in the related art.
When an optical pickup is capable of writing data in a DVD, a polarization optical system is used to increase the illumination efficiency. Namely, in the path from a laser diode (LD) 1, which serves as a light source, to an object lens (OL) 2, a polarized beam splitter (PBS) 3 is arranged, allowing the linearly-polarized light having the same polarization plane as the polarization plane of the linearly-polarized light from the LASER DIODE 1 to pass through. A ¼ wave plate 4 is arranged in front of the polarized beam splitter 3 to convert the linearly-polarized light to circularly-polarized light. This circularly-polarized light is condensed on the object lens 2, and is irradiated on a recording layer under a substrate of an optical recording medium 5 (simply referred to as an “optical disk” below).
The light reflected from the reflection surface of the optical disk 5 is converted into circularly-polarized light in a rotating polarization direction opposite to the rotating polarization direction of the incident circularly-polarized light. Passing through the ¼ wave plate 4, the reflected circularly-polarized light is converted to linearly-polarized light having a polarization direction perpendicular to the polarization plane of the laser diode 1, and is reflected by the polarized beam splitter 3 so as to be directed to a photo detector (PD) 6.
If the ¼ wave plate 4 generates a perfect circularly-polarized laser beam, the light transmitting through polarized beam splitter 3, namely, the light returning to the laser diode 1 becomes “zero”, and the reflected light from the optical disk 5 is totally detected by the photo detector 6.
Such an optical pickup scans fine recording marks on the optical disk 5 with a laser beam having a beam size reduced by the object lens 2 to reproduce the recorded data on the optical disk 5. In this process, on the back side of the optical disk 5, because of surface vertical runout, the data recording surface may be tilted relative to the laser beam from the optical pickup. In this case, the laser beam for reproducing data in the optical disk 5 ends up being incident on the data recording surface at an inclined angle (not orthogonal). Hence, the light spot produced on the data recording surface involves comma aberration, and as a result, the spot shape becomes asymmetric. Furthermore, it becomes difficult to precisely read the recorded data.
Considering the Blu-ray Disc, in order to obtain a very large capacity, the light spot should be greatly reduced, and for this purpose, the numerical aperture of the objective lens 2 should be reduced. In this case, because of the tilt of the optical disk 5, comma aberration increases, and the margin of the tilt of the optical disk 5 becomes small. For this reason, it is required to compensate for the tilt of the optical disk 5 to realize a very large capacity.
Japanese Laid-Open Patent Application No. 2002-260264 (hereinafter, refer to as reference 1) discloses a technique of tilt compensation by using a four-axial actuator (ACT).
FIG. 16 is a schematic view illustrating an arrangement of the object lens 2 and the four-axial actuator 7, and the size of the light spot on an optical disk under normal tilt driving condition in the related art.
As shown in FIG. 16, usually the object lens 2 is arranged parallel to the optical disk 5, and is supported by the wires of the four-axial actuator 7.
Under this condition, if an incident light beam is focused on the recording surface of the optical disk 5, and if viewing the shape of the light spot, one finds a round light spot as illustrated in a part of the optical disk 5 shown in the upper portion in FIG. 16.
FIG. 17 is a schematic view illustrating an arrangement of the object lens 2 and the four-axial actuator 7 and the size of the light spot on an optical disk 5 under abnormal tilt driving conditions in the related art.
However, if the optical disk 5 is tilted, as shown in FIG. 17, the light spot becomes an elliptical shape as illustrated in a part of the optical disk 5 shown in the upper portion in FIG. 17.
FIG. 18 is a schematic view illustrating an arrangement of the object lens 2 and the four-axial actuator 7 and the size of the light spot on an optical disk in the course of tilt compensation in the related art.
To solve the problems as shown in FIG. 17, as shown in FIG. 18, the object lens 2 is tilted by the four-axial actuator 7 to maintain the object lens 2 parallel to the optical disk 5.
Under this condition, a round light spot is obtained again as illustrated in a part of the optical disk 5 shown in the upper portion in FIG. 18.
Japanese Laid-Open Patent Application No. 2003-016677 (hereinafter, refer to as reference 2) discloses a technique of tilt compensation as described below. Specifically, at an initial position of the four-axial actuator relative to the object lens, if the optical axis of a laser beam is not perpendicular to the recording surface of the optical disk, or the tilt servo is executed when there is little reflected light during focus control, the tile error signal becomes unstable, the object lens is displaced, and the focus servo cannot be successful.
Upon this, as described in reference 2, the tilt control of the object lens is stabilized, specifically, the tilt control is performed ensuring the optical axis of a laser beam to be perpendicular to the recording surface of the optical disk, so that the tilt control of the optical disk is based on the tilt control of the object lens using the four-axial actuator to follow the warpage of the optical disk.
In addition, Japanese Laid-Open Patent Application No. 11-144280 (hereinafter, refer to as reference 3) discloses an optical disk device in which a tilt sensor is arranged on an object lens holder of an actuator to detect the relative tilt between the object lens and the optical disk. An object lens tilt driving quantity is calculated from the relative tilt between the object lens and the optical disk, and the actuator is driven by the quantity of the object lens tilt driving, thereby compensating for the optical disk tilt. Particularly, the optical disk device includes a servo signal calculation unit for calculating a servo signal such that the inclined angle between the object lens and the optical disk is an integer n (n is not 1).
However, in the tilt compensation system by using the above mentioned four-axial actuator, the tilt of the optical disk is detected, and the inclination of the object lens is to compensate for the tilt by a quantity exactly equaling to the detected tilt. In this method, the optical aberration depends on the shape of the object lens, and in most cases, the tilt angle of the lens resulting in minimum aberration is not equal to the tilt angle of the disk. Namely, even when the optical disk is maintained to be perpendicular to the optical axis of the object lens, sometimes, the aberration cannot be reduced.
In addition, when designing the shape of a lens, although it is possible to make the optical aberration minimum by equalizing the tilt angle of the lens with the tilt angle of the disk, in such a design, the wavelength has to be limited.
For example, when designing an object lens supporting three wavelengths of the Blu-ray Disc, DVD, and CD, for example, if it is designed that the optical comma aberration becomes the minimum when the optical disk is perpendicular to the optical axis of the object lens relative to the tilt angle of the optical disk at the wavelength of the Blu-ray Disc, the tilt angle of the lens resulting in minimum comma aberration is not equal to the tilt angle of the optical disk at the wavelengths of DVD and CD. Actually, it is difficult to make a design such that comma aberration is a minimum when the tilt angle of the lens is equal to the tilt angle of the optical disk at all wavelengths.
In addition, with an object lens working at a single wavelength, even when it is designed that the comma aberration is minimum when the tilt angle of the lens is equal to the tilt angle of the optical disk, due to change with time, temperature characteristics, and other factors, the tilt angle of the lens resulting from minimum comma aberration relative to the tilt angle of the optical disk changes gradually.
As disclosed in reference 3, when performing the object tilt control by using a servo signal calculation unit for calculating a servo signal such that the inclined angle of the object lens holder relative to the object disk is an integer n (n is not 1), the reference of the angle is the optical axis. However, the output value from the optical disk tilt sensor attached to the object lens holder gives a relative tilt between the object lens and the optical disk, but cannot give the optical disk tilt relative to the optical axis directly.
Thus, one has formula (1).θe=(θ1+θ2)×n−θ2
Where θe represents a tilt error, θ1 represents the inclined angle of the object disk relative to the object lens holder, θ2 represents the inclined angle of a lens actuator, n represents a constant defined by the wavelength of a laser beam, the beam size, the numerical aperture (NA), and the lens shape.
The formula (1) is quite complicated, and it is difficult to constitute the calculation circuit.
In formula (1), n is a parameter defined by a wavelength of a laser beam, a beam size, the numerical aperture (NA), and the lens shape. But this parameter n is not described specifically in reference 3. For example, preferable values of n are not described.